Detective Loki's tattoos, Freemason ring and facial tics were all Jake Gyllenhaal's ideas.

Jake Gyllenhaal and Denis Villeneuve had such a great time doing Enemy (2013) that Villeneuve cast him for this movie without any audition.

To play Holly Jones, Melissa Leo wore a grey wig and a foam rubber posterior prepared by Costume Designer Renée April in order to "ground" her character. Leo also requested the props assistants to avoid cleaning her glasses after each day's shooting.

According to Screenwriter Aaron Guzikowski, the original cut of the film was three hours.

Detective Loki is clearly visible in many scenes wearing a Freemasons ring on his left hand.

Ryan Gosling auditioned for the role of Detective Loki which later went to Jake Gyllenhaal, a frequent collaborator of director Denis Villeneuve (and a friendly professional rival of Gosling's, being that they often audition for the same roles). Villeneuve and Gosling would finally get their chance to work together in Blade Runner 2049 (2017).

The screenplay for this film was featured in the 2009 Blacklist; a list of the "most liked" unmade scripts of the year.

Mark Wahlberg and Christian Bale were set to star, with Bryan Singer directing, but the two actors opted to make The Fighter (2010) instead.

After filming had been completed, the costume and prop departments donated all of the clothing and props to Fodac Thrift Store in Tucker, Georgia.

In this film, a detective named Loki obsessively tries to find two kidnapped children. In Norse Mythology there is a tale (Loka Táttur) where the god Loki persistently tries to protect a child from a giant named Skrymir after Odin and Hönir had tried, and given up on the task.

Leonardo DiCaprio was attached to the project for a long time, but he dropped out.

Hugh Jackman portrays a father whose daughter is kidnapped and missing in this film. He was originally attached to play a similar role in The Lovely Bones (2009), but dropped out. That role was filled by Mark Wahlberg, who at one point was attached to play the lead role in this film.

The name of Alex Jones' aunt is Holly Jones. Holly Jones is also the name of a ten-year old girl who was kidnapped and murdered in Toronto in 2003.

When detective Loki is looking through the notebook he finds at Bob Taylor's, the last page you see has a character drawn on it that looks a lot like the rabbit from Jake Gyllenhaal's movie Donnie Darko (2001).

Hugh Jackman was attached to the project with Antoine Fuqua directing. Both dropped out. After several years in development, Jackman returned in the lead role.

According to screenwriter Aaron Guzikowski, Detective Loki grew up in a boys home and didn't really have a family. He's a prisoner of his own past with his own demons.

Detective Loki's cards reveal his full name to be David Wayne Loki.

The byline of one of the newspaper articles shown in the film is "Aaron Guzikowski", the film's Screenwriter.

There are no opening credits, just the title of the movie.

(At about 1 h 25 min) Detective Loki finds an article about the death of Keller's father, William Dover. The article reads as follows: HEADLINE: Graterford Guard Commits Suicide In Home ARTICLE: A Pennsylvania State correction officer was found dead Monday morning in his Conyers apartment, apparently of a self-inflicted gunshot wound to the head, the police reported. The correction officer, William Dover, 46 years old, was found by his wife, Mary and his teenage son, in their apartment at 234 Campello Street. No suicide note was found, the police said. Officials at Graterford prison where Mr. Dover had worked for the past fourteen years declined to comment.

The film cast includes two Oscar winners: Melissa Leo and Viola Davis; and three Oscar nominees: Hugh Jackman, Jake Gyllenhaal, and Terrence Howard.

Detective Loki (Jake Gyllenhaal) has zodiacal sign symbols tattooed on his right hand fingers (Leo, Scorpio, Aries, Virgo). Jake Gyllenhaal starred in Zodiac (2007) directed by David Fincher.

When Keller Dover (Hugh Jackman) finds a newspaper in Holly Jones (Alex's aunt's) house the articles author stated is a field reporter Aaron Gruzikowski - name of the screenwriter of the film.

Jessica Chastain was rumored for a role.

Three of the main actors in the film have been seen in Marvel movies. Hugh Jackman famously plays Wolverine/Logan in the X-Men Series, Terrence Howard starred as Rodey in the original 2008 Iron Man (before being replaced by Don Cheadle in the films sequels), and Jake Gylenhall is set to play Mysterio in the upcoming Spider-Man: Far From Home. In addition, Viola Davis has appeared in the DCEU as Amanda Waller in Suicide Squad.

Jackman's character (Dover) owns a home that was handed down through his deceased father, it's location is on "Campello Street." The films screenwriter (Aaron Guzikowski) was born in Brockton, MA, which has a prominent neighborhood known as "Campello."

The moment where Holly asks Keller to drink a little more was improvised by Melissa Leo, who thought Hugh Jackman wasn't drinking enough to make the situation believable.

The first time it was submitted to the MPAA, it received an NC-17 rating, due to its tone and subject matter. The film's torture scenes were later cut by a couple of frames, along with scenes suggesting pedophilia, and it then received the R-rating.

Originally, Hugh Jackman was never discovered by police at end. He would have remained a missing person, and died a la The Vanishing (1988), but this was deemed too bleak.

Paul Dano admits to being attracted to roles of this nature (learning difficulties, screaming, nonsensical ranting) and says he draws on his own personal and private experiences to perfect these recurring roles.

For the hammer scene, Hugh Jackman was asked by Denis Villeneuve to be completely ferocious for the next take. That take was used in the final film.

Keller, the name of Hugh Jackman's character, is a German name that means basement or cellar. A couple of basements are seen in the film, including Keller's, which is full of survival supplies. Keller himself ends up trapped in a cellar.